Microfluidic systems have great potential for use in a clinical laboratory setting. However, these devices are limited by the fact that they have capacity for only very small sample volumes, typically on the order of a few microliters or less. When substances to be analyzed are found in a sample at a very low concentration, sensitivity can be limited. One way to overcome this limitation is by using an analyte amplification step to increase the analyte concentration either before or after introduction of the sample to a microfluidic device. For example, very small amounts of nucleic acids can be amplified using methods such as PCR. However, not all analytes can be amplified and, even when possible, amplification may require additional reagents and increase the complexity of analysis. New methods that allow analytes from large volume samples to be assayed in a microfluidic format and/or allow analyses without an amplification step would be valuable for clinical and other laboratory assays. The present invention meets this and many other needs.